Process

ABSTRACT

The present invention relates to a process for electrochemical extraction of a metal (M) from a metal (M) oxide, to a conducting electrode and to an electrolytic cell comprising the conducting electrode.

The present invention relates to a process for electrochemicalextraction of a metal (M) from a metal (M) oxide, to a conductingelectrode and to an electrolytic cell comprising the conductingelectrode.

In recent years, the direct electrochemical reduction of TiO₂ to Timetal in molten CaCl₂ has stimulated significant scientific andindustrial interest (see for example G. Z. Chen, et al, Nature 407(6802), 361 (2000); R. O, Suzuki and K. Ono, in Molten Salts Xiii,edited by P. C. Trulove, H. C. DeLong, R. A. Mantz et al. (2002), Vol.2002, pp. 810; T. H. Okabe et al, Journal of Alloys and Compounds 364(1-2), 156 (2004); S. L. Wang and Y. J. Li, Journal of ElectroanalyticalChemistry 571 (1), 37 (2004); and T. Nohira et al, Nature Materials 2(6), 397 (2003)). However the formation of a stable perovskite phase inthe intermediate stage of reduction hinders the diffusion of O²⁻ ionsforming a layered structure in the pellet which slows the overallkinetics (D. T. L. Alexander et al, Acta Materialia 54 (11), 2933 (2006)and K. Jiang et al., Angewandte Chemie-International Edition 45 (3), 428(2006)).

The conventional FFC process is of the type disclosed in WO-A-99/64638for the formation of Ti metal from a TiO₂ pellet cathode using a carbonrod anode in a molten bath of CaCl₂ at 900° C. at a constant voltage of3.1V in an argon atmosphere. The FFC process involves severalintermediate steps one of which includes the formation of stableperovskite phases (Alexander [supra] and C. Schwandt and D. J. Fray,Electrochimica Acta 51 (1), 66 (2005)). The formation of perovskite notonly reduces the diffusion of O²⁻ ions but also due to larger grain sizereduces the pore diffusion of CaCl₂ in the pellet. Although Jiang[supra] and R. Lilia Centeno-Sanchez et al, Journal of materials science42, 7494 (2007) showed that an increase in porosity could be achieved byadding carbon and polyethylene precursors to the pellet or by directlystarting from perovskite, the process still takes more than 24 hours toproduce Ti with 3000 ppm by weight of oxygen. It is evident thatdiffusion of O²⁻ ions is one of the limiting steps in the overallreduction of oxides in the FFC process. The low rate of reductionhinders the process being scaled-up and full metallisation is difficultto attain even with a small pellet. These limitations are a barrier to acontinuous process and render the FFC process solely a batch process.

The present invention is based on the recognition that the presence ofan alkali metal oxide (or a salt from which an alkali metal oxide can bederived) serves to increase the rate of electrochemical reduction of ametal oxide in an oxygen-dissolving molten electrolyte.

Viewed from a first aspect the present invention provides a process forelectrochemical extraction of a metal (M) from a metal (M) oxidecomprising:

applying a voltage between a cathode comprising (or consistingessentially of) or in contact with the metal (M) oxide and an anode inan oxygen-dissolving molten electrolyte in the presence of an alkalimetal (M^(a)) oxide whereby to form an alkali metal (M^(a)) metallate(M) phase.

In accordance with the process of the invention, alkali metal (M^(a))ions improve the diffusivity of oxygen by forming the alkali metal(M^(a)) metallate (M) phase. By way of example, where the alkali (M^(a))metal oxide is potassium oxide and the metal (M) oxide is TiO₂, TiO₂ isreduced to nearly 100% Ti metal with 1350 ppm of oxygen in less than 20hours. The presence of a potassium titanate (K₄TiO₄) liquid phaseprovides an efficient O²⁻ ion transport medium which substantiallyshortens the Ti production time. This opens up the possibility ofcontinuous Ti production at lower cost and therefore the more widespreadexploitation of Ti in consumer products.

The alkali metal (M^(a)) oxide may be a caesium, rubidium, lithium,sodium or potassium oxide. Preferably the alkali metal (M^(a)) oxide islithium, sodium or potassium oxide. Particularly preferably the alkalimetal (M^(a)) oxide is potassium oxide.

The alkali metal (M^(a)) oxide may be an additive or may be formed insitu by decomposition of a decomposable alkali metal (M^(a)) salt intothe alkali metal (M^(a)) oxide. Preferably the alkali metal (M^(a))oxide forms the alkali metal (M^(a)) metallate (M) phase from a reactionof the alkali metal (M^(a)) oxide with a metal (M″) metallate (M) phase.Particularly preferably the metal (M″) metallate (M) phase is a solidphase. Particularly preferably the metal (M″) metallate (M) phase is aperovskite (or perovskite-type) phase. Preferably M″ is an alkalineearth metal, particularly preferably Ca, Sr or Ba, most preferably Ca.

Preferably the diffusivity of oxygen in the alkali metal (M^(a))metallate (M) phase is higher than the diffusivity of oxygen in themetal (M″) metallate (M) phase.

Preferably the alkali metal (M^(a)) metallate (M) phase is a liquid.Preferably the alkali metal (M^(a)) metallate (M) phase is atransitional phase.

In a preferred embodiment, the alkali metal (M^(a)) oxide is anadditive. The alkali metal (M^(a)) oxide may be added (e.g. in the formof a powder) to the oxygen-dissolving molten electrolyte.

Preferably the alkali metal (M^(a)) oxide is in admixture with the metal(M) oxide in or in contact with the cathode. The mixture of alkali metal(M^(a)) oxide and metal (M) oxide may be solid or liquid (eg molten).

Preferably the process of the invention further comprises: mixing thealkali metal (M^(a)) oxide and the metal (M) oxide. Particularlypreferably the process of the invention further comprises: forming themixture of alkali metal (M^(a)) oxide and metal (M) oxide into aself-supporting mixture (eg a pellet, slab, sheet, wire, foil, basket ortube). The forming step may be pressing. The self-supporting mixture maybe the cathode or may be contactable with the cathode. Preferably theself-supporting mixture is a pellet. The mixing step may be followed byheat treating the mixture.

The alkali metal (M^(a)) oxide may be present in the self-supportingmixture in an amount in excess of a trace amount, preferably in excessof 5 wt %, particularly preferably in excess of 10 wt %, more preferablyin excess of 20 wt %. Preferably the alkali metal (M^(a)) oxide ispresent in the self-supporting mixture in an amount in the range 10-70wt %, particularly preferably 20-50 wt %.

In a preferred embodiment, the alkali metal (M^(a)) oxide is formed insitu by decomposition of a decomposable alkali metal (M^(a)) salt. Thedecomposable alkali metal (M^(a)) salt may be thermally decomposable.The decomposable alkali metal (M^(a)) salt may be added (e.g. in theform of a powder) to the oxygen-dissolving molten electrolyte

Preferably the decomposable alkali metal (M^(a)) salt is in admixturewith the metal (M) oxide in or in contact with the cathode.

Preferably the process of the invention further comprises: mixing thedecomposable alkali metal (M^(a)) salt and the metal (M) oxide.Particularly preferably the process of the invention further comprises:forming the mixture of decomposable alkali metal (M^(a)) salt and metal(M) oxide into a self-supporting mixture (eg a pellet, slab, sheet,wire, basket, foil or tube). The forming step may be pressing. Theself-supporting mixture may be the cathode or may be contactable withthe cathode. Preferably the self-supporting mixture is a pellet. Themixing step may be followed by heat treating the mixture.

The decomposable alkali metal (M^(a)) salt may be present in theself-supporting mixture in an amount in excess of a trace amount,preferably in excess of 5 wt %, particularly preferably in excess of 10wt %, more preferably in excess of 20 wt %. Preferably the decomposablealkali metal (M^(a)) salt is present in the self-supporting mixture inan amount in the range 10-70 wt %, particularly preferably 20-50 wt %.

Preferably the decomposable alkali metal (M^(a)) salt is decomposableinto one or more gaseous species. The gaseous species may be selectedfrom the group consisting of water and carbon dioxide. Decomposition ofthe alkali metal (M^(a)) salt into one or more gaseous species mayadvantageously promote electrochemical reduction by forming porositywithin the cathode. Continuous formation of pores permits fast transportof molten electrolyte species (e.g. CaO and CaCl₂) which accelerateschemical reduction.

The decomposable alkali metal (M^(a)) salt may be an alkali metal(M^(a)) halide, carbonate, bicarbonate, hydrogen sulphide, hydrogensulphate, nitrate, chlorate or sulphate. Preferably the decomposablealkali metal (M^(a)) salt is an alkali metal (M^(a)) bicarbonate.

The decomposable alkali metal (M^(a)) salt may be a caesium, rubidium,lithium, sodium or potassium salt. Preferably the decomposable alkalimetal (M^(a)) salt is a lithium, sodium or potassium salt. Particularlypreferably the decomposable alkali metal (M^(a)) salt is a potassiumsalt, more preferably KCl.

The metal (M) may be a reactive metal element, semi-metal element, metalalloy or metalloid element.

In a preferred embodiment, the metal (M) forms a solid perovskite (orperovskite-type) phase in the oxygen-dissolving molten electrolyte. Thesolid perovskite phase may be an alkaline earth metal (e.g. Ca)metallate (M) phase.

The metal (M) may be one or more metals selected from the groupconsisting of group HA metals, group IIIA metals, group IVA metals,group B transition metals, rare earth metals and alloys thereof.Preferably the metal (M) is one or more metals selected from the groupconsisting of Mg, Al, Si, Ge, group IVB transition metals, group VBtransition metals, group VIB transition metals, group VIIB transitionmetals, group VIIIB transition metals, lanthanides, actinides and alloysthereof. Particularly preferably the metal (M) is one or more metalsselected from the group consisting of group IVB transition metals, groupVB transition metals, group VIB transition metals, group VIIIBtransition metals, actinides and alloys thereof. Especially preferablythe metal (M) is one or more metals selected from the group consistingof Ti, Nb, Ta, U, Th, Cr, Fe, steel and Zr. More especially preferred isone or more metals selected from the group consisting of Ti, Nb, Ta andZr. Most preferred is Ti.

The alkali metal (M^(a)) metallate (M) phase may be M^(a) ₂MO₃ or M^(a)₄MO₄. Preferred is M^(a) ₄MO₄. For example, where M is titanium, thepreferred phase is M^(a) ₄TiO₄.

The metal (M) oxide may be the cathode or the metal (M) oxide inadmixture with either the alkali metal (M^(a)) oxide or the alkali metal(M^(a)) salt decomposable into the alkali metal (M^(a)) oxide may be thecathode. Preferably the metal (M) oxide in admixture with either thealkali metal (M^(a)) oxide or the alkali metal (M^(a)) salt decomposableinto the alkali metal (M^(a)) oxide is the cathode.

Alternatively the metal (M) oxide may be in contact with a cathode. Inthis embodiment, the metal (M) oxide may be self-supporting (e.g. in theform of a pellet) and the cathode may be a bath, crucible or basket(e.g. a perforated basket). Alternatively the metal (M) oxide may be inadmixture with the alkali metal (M^(a)) oxide or the alkali metal(M^(a)) salt decomposable into the alkali metal (M^(a)) oxide. Themixture may be a self-supporting mixture (e.g. in the form of a pelletor a perforated basket) or a molten mixture. Where the mixture is amolten mixture, the cathode is preferably a crucible. The crucible maybe composed of a metal such as titanium or a titanium alloy and thisembodiment advantageously prevents contamination of theoxygen-dissolving molten electrolyte.

Alternatively the metal (M) oxide may be in the oxygen-dissolving moltenelectrolyte in contact with the cathode. The alkali metal (M^(a)) oxideor the alkali metal (M^(a)) salt decomposable into the alkali metal(M^(a)) oxide may be in the oxygen-dissolving molten electrolyte incontact with the cathode. The cathode may be a metal substrate such assteel which may be in the form of a cathodic bath, crucible, basket orone or more pellets.

The oxygen-dissolving molten electrolyte may be (or contain) a compoundof an alkaline earth metal (e.g. Ca, Sr or Ba), Li, Cs or Y (or amixture thereof). Preferably the oxygen-dissolving molten electrolyte isa compound of Ca. The oxygen-dissolving molten electrolyte may be (orcontain) a halide. Preferably the molten electrolyte contains (egconsists essentially of) CaCl₂. Particularly preferably the moltenelectrolyte contains CaCl₂ and an alkali metal halide (preferably achloride). Preferred is a mixture of CaCl₂ and KCl or of CaCl₂ and LiCl.

The anode may be carbon (e.g. graphite).

Typically in the process of the invention the anode is an inert anode.Preferred is an anode which is substantially unreactive with oxygen.Preferred is an anode which is substantially insoluble in the moltenelectrolyte.

Typically the inert anode is a non-carbon anode. Preferred is an inertmetal alloy anode. An inert metal alloy anode advantageously provideseffective current efficiency.

Preferably the anode is composed of an Al-E-Cu based alloy comprising anintermetallic phase of formula:

Al_(x)E_(y)Cu_(z)

wherein:

E denotes one or more metallic elements;

x is an integer in the range 1 to 5;

y is an integer being 1 or 2; and

z is an integer being 1 or 2.

The Al-E-Cu based alloy may be substantially monophasic or multiphasic.Preferably the intermetallic phase is present in the Al-E-Cu based alloyin an amount of 50 wt % or more (eg in the range 50 to 99 wt %).Preferably the Al-E-Cu based alloy further comprises an orderedhigh-temperature intermetallic phase of E with aluminium, particularlypreferably Al₃E. Other intermetallic phases may be present.

In a preferred embodiment, the Al-E-Cu based alloy is substantially freeof CuAl₂. This is advantageous because CuAl₂ has a tendency to melt atthe elevated temperatures which are deployed typically in the process ofthe invention. Preferably CuAl₂ is complexed.

In a preferred embodiment, the Al-E-Cu based alloy falls other than onthe E poor side of the tie line joining Al₃E and ECu₄ (e.g. on the Erich side of the tie line joining Al₃E and ECu₄).

In a preferred embodiment, the Al-E-Cu based alloy comprises anintermetallic phase falling on or near to the tie line joining Al₃E andECu₄.

In a preferred embodiment, the Al-E-Cu based alloy falls other than onthe E poor side of the tie line joining Al₃E and AlECu₂ (eg on the Erich side of the tie line joining Al₃E and AlECu₂).

In a preferred embodiment, the Al-E-Cu based alloy comprises anintermetallic phase falling on or near to the tie line joining Al₃E andAlECu₂.

In a preferred embodiment, the Al-E-Cu based alloy falls other than onthe E poor side of the ξ, Al₅E₂Cu, EAlCu₂ and β-ECu₄ phase tie line(wherein ξ is a phase falling between Al₃Ti and Al₂Ti with 3 at % orless of Cu (e.g. 2-3 at % Cu)).

In a preferred embodiment, the Al-E-Cu based alloy comprises anintermetallic phase falling on or near to the ξ, Al₅E₂Cu, EAlCu₂ andβ-ECu₄ phase tie line.

Preferably the intermetallic phase is Al₅E₂Cu. Particularly preferablythe Al-E-Cu based alloy further comprises Al₃E.

Preferably the intermetallic phase is EAlCu₂. Particularly preferablythe Al-E-Cu based alloy further comprises β-ECu₄

The anode may be composed of a homogenous, partially homogenous ornon-homogeneous Al-E-Cu based alloy.

Typically E has a potential in the anode which is lower than it would bein the molten electrode.

In a preferred embodiment, the anode develops a passivating layer.Preferably the passivating layer withstands oxidation in anodicconditions.

In a preferred embodiment, E is a single metallic element. The singlemetallic element is preferably Ti.

In an alternative preferred embodiment, E is a plurality (eg two, three,four, five, six or seven) of metallic elements. In this embodiment, afirst metallic element is preferably Ti. Typically the first metallicelement of the plurality of metallic elements is present in asubstantially higher amount than the other metallic elements of theplurality of metallic elements. Each of the other metallic elements maybe present in a trace amount. Each of the other metallic elements may bea dopant. Each of the other metallic elements may substitute Al, Cu orthe first metallic element. The presence of the other metallic elementsmay improve the high-temperature stability of the alloy (eg from 1200°C. to 1400° C.).

In a preferred embodiment, E is a pair of metallic elements. In thisembodiment, a first metallic element is preferably Ti. Typically thefirst metallic element of the pair of metallic elements is present in asubstantially higher amount than a second metallic element of the pairof metallic elements (eg in a weight ratio of about 9:1). The secondmetallic element may be present in a trace amount. The second metallicelement may be a dopant. The second metallic element may substitute Al,Cu or the first metallic element. The presence of a second metallicelement may improve the high-temperature stability of the alloy (e.g.from 1200° C. to 1400° C.).

Preferably the pair of metallic elements has similar atomic radii.Preferably the atomic radius of the second metallic element is similarto the atomic radius of Cu. Preferably the atomic radius of the secondmetallic element is similar to the atomic radius of Al.

In a preferred embodiment, E is one or more of the group consisting ofgroup B transition metal elements (e.g. first row group B transitionmetal elements) and lanthanide elements. Preferably E is one or moregroup IVB, VB, VIIB, VIIB or VIIIB transition metal elements,particularly preferably one or more group IVB, VIIB or VIIIB transitionmetal elements.

In a preferred embodiment, E is one or more metallic elements of valencyII, III, IV or V, preferably II, III or IV.

In a preferred embodiment, E is one or more metallic elements selectedfrom the group consisting of Ru, Ti, Zr, Cr, Nb, V, Co, Ta, Fe, Ni, Laand Mn. In a particularly preferred embodiment, E is one or moremetallic elements selected from the group consisting of Ti, Fe, Cr andNi.

Preferably E is or includes a metallic element capable of reducing thetendency of CuAl₂ towards grain boundary segregation at an elevatedtemperature. In this embodiment, the metallic element capable ofreducing the tendency of CuAl₂ towards grain boundary segregation at anelevated temperature may be the second metallic element of a plurality(e.g. a pair) of metallic elements. Particularly preferably E is orincludes a metallic element capable of forming a complex with CuAl₂.Preferred metallic elements for this purpose are selected from the groupconsisting of Fe, Ni and Cr, particularly preferably Ni and Fe,especially preferably Ni.

Preferably E is or includes a metallic element capable of reducing thetendency of the first metallic element or Cu to dissolve in moltenextractant. In this embodiment, the metallic element may be the secondmetallic element of a plurality (eg a pair) of metallic elements.Preferred metallic elements for this purpose are selected from the groupconsisting of Fe, Ni, Co, Mn and Cr, particularly preferably the groupconsisting of Fe and Ni (optionally together with Cr).

Preferably E is or includes a metallic element capable of promoting thepassivation of the surface of the anode in the presence of aoxygen-dissolving molten electrolyte. For this purpose, the metallicelement may form or stabilise an oxide film. In this embodiment, themetallic element may be the second metallic element of a plurality (e.g.a pair) of metallic elements. Preferred metallic elements for thispurpose are selected from the group consisting of Ru, Fe, Ni and Cr.Particularly preferably E is Ti, Fe, Ni and Cr in which the formation ofa combination of oxides such as iron oxides, chromium oxides, nickeloxides and alumina advantageously promotes passivation.

Preferably E is or includes a metallic element selected from the groupconsisting of Zr, Nb and V. Particularly preferred is V or Nb. Thesesecond metallic elements are advantageously strong intermetallicformers. In this embodiment, the metallic element is the second metallicelement of a plurality (eg a pair) of metallic elements.

Preferably E is or includes a metallic element capable of forming anordered high-temperature intermetallic phase with aluminium metal.Particularly preferably E is or includes a metallic element capable offorming Al₃E.

Preferably E is or includes Ti. A titanium containing alloy typicallyhas electrical resistivity in the range 3 to 15 μohm cm at roomtemperature.

Preferably the intermetallic phase is Al₅Ti₂Cu. Particularly preferablythe Al—Ti—Cu based alloy further comprises Al₃Ti.

Preferably the intermetallic phase is TiAlCu₂. Particularly preferablythe Al—Ti—Cu based alloy further comprises β-TiCu₄

In a preferred embodiment, E is or includes Ti and a second metallicelement selected from the group consisting of Fe, Cr, Ni, V, La, Nb andZr, preferably the group consisting of Fe, Cr and Ni. The secondmetallic element advantageously serves to enhance high-temperaturestability of the Al—Ti—Cu phases.

The anode may be composed of an Al-E-Cu based alloy obtainable byprocessing a mixture of 35 atomic % Al or more (preferably 50 atomic %Al or more), 35 atomic % E or more (wherein E is a first metallicelement as hereinbefore defined) and a balance of Cu and optionally E′(wherein E′ is one or more of the additional metallic elementshereinbefore defined).

In a preferred embodiment, the anode is composed of an Al-E-Cu basedalloy obtainable by processing a mixture of (65+x) atomic % Al, (20+y)atomic % E (wherein E is a first metallic element as hereinbeforedefined) and (15-x-y) atomic % Cu, optionally together with z atomic %of E′ (wherein E′ is one or more of the additional metallic elementshereinbefore defined) wherein E′ substitutes Cu, Al or E.

In this embodiment, the alloy may be obtainable by casting, preferablyin an oxygen deficient atmosphere (eg an inert atmosphere). For example,a mixture may be melted in an argon-arc furnace under an atmosphere ofargon gas and then solidified in an argon atmosphere. Alternatively inthis embodiment, the alloy may be obtainable by flux-assisted melting,vacuum arc or vacuum melting using a resistance furnace. Contaminationby O, C, N, S or P should be minimised.

In a preferred embodiment, the anode is at least as conducting atelevated temperature (e.g. at 900° C.) as a carbon electrode. Preferablythe anode is more conducting at elevated temperature (e.g. at 900° C.)than a carbon electrode.

In a preferred embodiment, the decomposable alkali metal (M^(a)) saltmay be present with an amount of endogenous hydroxide ions. A hydroxideion decomposes at the cathode into an oxide ion (which moves to theanode) and a proton. At the cathode, this leads to the formation ofoccluded hydrogen in the metal (M) which may react with oxygen (forexample in subsequent steps such as remelting) to advantageously lowerthe oxygen content of the metal (M) (e.g. to a level as low as 1100ppm). A hydrogenated metal (M) (e.g. hydrogenated uranium) produced inthis embodiment is useful. For example, a hydrogenated metal (M) may bea useful hydrogen storage material. The hydrogen may be removed by (forexample) plasma melting.

In a preferred embodiment, the decomposable alkali metal (M^(a)) saltmay be present with an amount of exogenous hydroxide ions. Preferablythe exogenous hydroxide ions are provided by an alkaline additive. Thealkaline additive may be an alkali metal hydroxide (such as lithium,sodium or potassium hydroxide), an alkali metal hydride (such aslithium, sodium or potassium hydride) or an alkaline earth metalhydroxide. The alkaline additive may be added to the oxygen-dissolvingmolten electrolyte.

The process of the invention may be carried out at an elevatedtemperature typically in the range 600-1000° C., preferably 850-1000° C.(e.g. about 900° C.).

The process of the invention for a discrete batch of metal (M) oxide maybe carried out to substantially complete conversion over a period ofless than 20 hours, preferably less than 10 hours (e.g. 8 hours),particularly preferably less than 4 hours. This advantageously minimisesenergy input and therefore costs.

The voltage is typically less than the discharge potential of metals inthe oxygen-dissolving molten electrolyte. For example, the voltage maybe less than 3.5V (eg about 3.0V).

In a preferred embodiment, the process of the invention is carried outin an oxygen deficient atmosphere (eg an inert atmosphere such asargon).

The process of the invention typically achieves a rate of metal (M)extraction of 99% or more, preferably 99.9% or more.

The process of the invention typically produces metal (M) with an oxygencontent of less than 2500 ppm O₂ by weight, preferably less than 1500ppm O₂ weight.

In a preferred embodiment, the process of the invention comprises:

applying a voltage between a cathode comprising TiO₂ in admixture withan alkali metal (M^(a)) salt decomposable into the alkali metal (M^(a))oxide and an anode in an oxygen-dissolving molten CaCl₂-containingelectrolyte whereby to form a liquid alkali metal (M^(a)) titanatephase.

In a preferred embodiment, the process of the invention furthercomprises:

measuring the current flow between the cathode and the inert metal alloyanode over a temporal range;

relating a characteristic of the current flow between the cathode andthe inert metal alloy anode over the temporal range to the extent ofelectrochemical extraction of the metal (M) from the metal (M) oxide.

Viewed from a further aspect the present invention provides a conductingelectrode comprising (or consisting essentially of) a metal (M) oxideand either an alkali metal (M^(a)) oxide capable of forming an alkalimetal (M^(a)) metallate (M) phase or an alkali metal (M^(a)) saltdecomposable into an alkali metal (M^(a)) oxide capable of forming analkali metal (M^(a)) metallate (M) phase.

The conducting electrode may (in use) be a cathode as hereinbeforedefined.

In a preferred embodiment, the conducting electrode comprises a metal(M) oxide and an alkali metal (M^(a)) salt decomposable into an alkalimetal (M^(a)) oxide capable of forming an alkali metal (M^(a)) metallate(M) phase.

Viewed from a still further aspect the present invention provides theuse of a conducting electrode or cathode as hereinbefore defined in anelectrolytic cell.

Viewed from an even still yet further aspect the present inventionprovides an electrolytic cell comprising a cathode which comprises or isin contact with a metal (M) oxide and one or more inert anodes incontact with a fusible or fused oxygen-dissolving electrolyte in thepresence of an alkali metal (M^(a)) oxide.

The (or each) inert anode may be as hereinbefore defined. The fusedoxygen-dissolving electrolyte may be an oxygen-dissolving moltenelectrolyte as hereinbefore defined. The cathode may be as hereinbeforedefined.

Generally the electrolytic cell is operated in an inert atmosphere (egan argon atmosphere). Preferably the fusible or fused oxygen-dissolvingelectrolyte comprises CaCl₂.

In a first preferred embodiment, the electrolytic cell comprises asingle inert anode. The alkali metal (M^(a)) oxide (eg K₂O) may bepresent in the fused oxygen-dissolving electrolyte. Preferably thecathode is a cathodic basket (eg a perforated basket) or crucible inwhich is carried the metal (M) oxide (eg in the form of a pellet).

The electrolytic cell may be a continuous cell.

In a second preferred embodiment, the cathode is a cathodic vessel whichis adapted to facilitate in use continuous flow of the fusedoxygen-dissolving electrolyte between a feeder end into which the fusedoxygen-dissolving electrolyte is feedable and a discharge end from whichthe fused electrolyte is dischargeable, wherein the electrolytic cellcomprises a plurality of inert anodes housed in the cathodic vesselbetween the feeder end and the discharge end.

Particularly preferably the electrolytic cell further comprises: acathodic separation vessel downstream from the discharge end, whereinthe cathodic separation vessel houses an inert anode.

Preferably the cathodic separation vessel houses a chlorine meter.

Preferably the cathodic separation vessel houses an oxygen meter.

Preferably the cathodic separation vessel comprises a referenceelectrode to assist in the measurement of current flow between thecathodic separation vessel and the inert anode. The current flow may beused to determine the extent of electrochemical reduction of the metal(M) oxide.

In the second preferred embodiment, the metal (M) oxide may be presentin the fused oxygen-dissolving electrolyte (eg in the form of asuspended powder or a pellet). The alkali metal (M^(a)) oxide (e.g. K₂O)may be present in the fused oxygen-dissolving electrolyte.

In a third preferred embodiment, the electrolytic cell comprises aplurality of inert anodes housed in a vessel which contains the fusedoxygen-dissolving electrolyte, wherein a mixture of the alkali metal(M^(a)) oxide and metal (M) oxide in contact with a cathode is presentin the form of a plurality of self-supporting elements conveyable in usethrough the fused oxygen-dissolving electrolyte.

Each self-supporting element may be a pellet or a basket (e.g. aperforated basket). The self-supporting elements may be mounted on aconveyor. The self-supporting elements may be dismountably mounted on aconveyor. The self-supporting elements may be conveyed in and out of thefused oxygen-dissolving electrolyte. The self-supporting elements may becirculatory (e.g. recirculatory).

In a fourth preferred embodiment, the electrolytic cell comprises aplurality of inert anodes housed in a vessel which contains the fusedoxygen-dissolving electrolyte, wherein the alkali metal (M^(a)) oxideand metal (M) oxide are present in the fused oxygen-dissolvingelectrolyte (e.g. in the form a suspension) in contact with a pluralityof cathodic elements conveyable in use through the fusedoxygen-dissolving electrolyte.

Each cathodic element may be a pellet. The cathodic elements may bemounted on a conveyor. The cathodic elements may be dismountably mountedon a conveyor. The cathodic elements may be conveyed in and out of thefused oxygen-dissolving electrolyte. The cathodic elements may becirculatory (eg recirculatory).

In a fifth preferred embodiment, the cathode is a metal cruciblecontaining the alkali metal (M^(a)) oxide and metal (M) oxide in moltenadmixture, wherein the metal crucible is suspended in the fusedoxygen-dissolving electrolyte. The metal crucible may be composed oftitanium metal or a titanium metal alloy. The fifth embodimentadvantageously prevents contamination of the fused oxygen-dissolvingelectrolyte by the molten admixture.

The present invention will now be described in a non-limitative sensewith reference to the Examples and accompanying Figures in which:

FIG. 1: K—Ti—O phase diagram at 1173K plotted using FACTSAGEthermodynamic software (C. Bale et al., FACTSAGE (Ecole PolytechniqueCRCT, Montreal, Quebec Canada));

FIG. 2: Elemental map of the cross section of a partially reacted TiO₂pellet after treatment according to an embodiment of the process of theinvention;

FIG. 3: Current vs time graph for the process according to the inventionat an applied voltage of 3.1V;

FIG. 4 a: Low magnification image of the cross section of a Ti pelletfully metallised in a LiCl—CaCl₂ molten bath;

FIG. 4 b: High magnification image of Ti metal obtained from the innerregion of the pellet seen in FIG. 3;

FIGS. 5 a and 5B: XRD of a TiO₂+KHCO₃ pellet roasted for 1 hour andelectrolysed for 0.5 hours (see FIG. 5 a) and 1 hour (see FIG. 5 b) in amolten bath of CaCl₂—LiCl showing phases of Ti (ICDD 5-682), CaTiO₃(ICDD 42-423), CaTi₂O₄ (ICDD 11-29) and TiO (ICDD 8-117);

FIG. 6: XRD of titanium metal formed after 20 hours of electrolysis;

FIG. 7: A schematic illustration of a first embodiment of theelectrolytic cell according to the invention;

FIG. 8: A schematic illustration of a second embodiment of theelectrolytic cell according to the invention;

FIG. 9: A schematic illustration of a third embodiment of theelectrolytic cell according to the invention;

FIG. 10: A schematic illustration of a fourth embodiment of theelectrolytic cell according to the invention; and

FIG. 11: A schematic illustration of a fifth embodiment of theelectrolytic cell according to the invention

EXAMPLE 1 Method

Pellets were prepared by mixing 1-2 g of TiO₂ with 0.2-0.5 g of KHCO₃ atdifferent weight ratios. In each case, the mixture was heat treated for1 hour at 1073K and pressed in a die at a pressure of 3643 atm. A holewas drilled in the pellet with a 2 mm drill bit. The pellet wassuspended in a steel electrode which acted as a cathode with amolybdenum wire. An Al—Ti—Cu intermetallic anode was suspended on asteel electrode with a molybdenum wire. The two electrodes wereconnected to a power supply which was set to a constant voltage of 3.1V.

Molten electrolytic mixtures of KCl—CaCl₂ and LiCl—CaCl₂ were preparedby taking 180 gms of CaCl₂ with 20 gms of KCl and LiCl respectively. Ineach case, the mixture was transferred into a zircon crucible which waslowered into a furnace maintained at 320° C. The mixture was heattreated for 24 hours and then transferred into an alumina crucible andheated to 800° C. at 0.5° C. per minute after which the temperature wasraised to 920° C. at a rate of 2° C. per minute. During heating, argongas was passed into the furnace at 500 ml min⁻¹. Once the electrolytewas fully molten, the temperature of the furnace was lowered to 900° C.The two electrodes were lowered into the furnace and a potential of 3.1Vwas applied using an Agilent 6651A DC power supply. The experiments werecarried out for a period of 8-24 hours.

Pellets were removed at intervals of 30 and 60 minutes of electrolysisand washed in water for 24 hours. The pellets were finely ground using amortar and pestle for X-ray powder diffraction analysis. The diffractionwas carried out using Cu—Kα as target at a scanning rate of 0.02° sec⁻¹.

Results

An increase in internal porosity was achieved readily in situ by thepresence of KHCO₃ in the TiO₂ pellet. As KHCO₃ decomposes, it producespotassium oxide, carbon dioxide and water. The liberated gaseous mixtureof CO₂ and H₂O increases the porosity in the pellet which enhances thecontact surface area between CaCl₂ and TiO₂ and facilitates rapidcathodic dissociation of TiO₂.

Besides pore formation, a much more significant reaction takes placebetween K₂O and CaTiO₃. K⁺ions diffuse into the perovskite lattice whichbreaks the structure by forming more stable liquid potassium titanatesas shown in equation [1] (ascertained from an equilibrium calculationperformed using FACTSAGE see Bale [supra]). The calcium oxide formed inthis reaction is dissolved in the molten salt bath until it reachessaturation:

CaTiO₃+2K₂O═K₄TiO₄ ⁺CaO ΔG=−334349.6 J mole⁻¹ at T=900° C.  [1]

As the diffusivity of O²⁻ ions in the liquid phase is faster than insolid CaTiO₃, the reduction of K₄TiO₄ to the Magneli phases through toTi metal occurs rapidly as no major reorganisation of crystalline TiO₂is required. The Magneli phases (Ti₄O₇, Ti₃O₅) all have a distortedrutile structure with a larger number of oxygen vacant sites. From aphase equilibrium analysis, it was established that the K₄TiO₄ liquidphase can be in equilibrium with the Magneli phase and continue to shiftthe equilibrium with the progression of reduction to the metallic phase(see FIG. 1). The formation of liquid phase increases the reactionkinetics which is evident as Ti metal was observed within the first halfan hour of electrolysis. The K⁺ ions produced from the decomposition ofpotassium titanate reacts with the molten electrolyte and forms KCaCl₃.

By controlling the volume of the liquid phase of potassium titanate, theloss of Ti in the molten salt can be prevented. If the liquid phasedrains out from the solid pellet into the CaCl₂ bath, TiO₂ is thenirreversibly lost into the CaCl₂ bath.

FIGS. 5 a and 5B are the XRD pattern of the pellet at 0.5 hours (seeFIGS. 5 a) and 1 hour (see FIG. 5 b) of electrolysis. Phases of Ti (ICDD5-682), CaTiO₃ (ICDD 42-423), CaTi₂O₄ (ICDD 11-29) and TiO (ICDD 8-117)are present. A comparison of FIGS. 5 a and 5 b shows that the perovskitepeak is suppressed as perovskite is decomposed. After 20 hours ofelectrolysis, the XRD pattern (FIG. 6) shows that titanium metal ispresent.

EXAMPLE 2

A number of experiments were carried out to change the ratio ofpotassium bicarbonate in the pellet in the range 10-50 wt %. Experimentswere also conducted on the two different types of molten salt containingCaCl₂—KCl and CaCl₂—LiCl mixtures at 900° C. with a constant voltage of3.1V. Both processes yielded complete reduction of TiO₂ pellet to Timetal. The residual concentration of oxygen dissolved in the Ti metalwas determined by X-ray diffraction analysis (see M. Dechamps et al.,Scripta Metallurgica 11 (11), 941 (1977)) and was found to be 1350 ppmby weight.

A first experiment was carried out with a pellet containing 20 wt %potassium bicarbonate in a CaCl₂—KCl bath for 8 hours. The elemental mapin FIG. 2 demonstrates the formation of Ti metal layer with a thicknessof 500 μm beyond which there is a high concentration of calcium,titanium, potassium and chlorine. From the elemental map in FIG. 2, itis possible to observe discrete regions of KCl within the CaCl₂ layerwhich can occur via reaction [2] at 900° C.:

K₂O+CaCl₂=CaO+KCl ΔG=−346968 J mole⁻¹  [2]

When the concentration of potassium bicarbonate was increased from 20 wt% to 50 wt % and electrolysis was performed for 20 hours, it was foundthat a uniform microstructure of Ti metal was formed across the crosssection of the pellet with the majority of the area being metallised.Furthermore when the salt bath was replaced by LiCl—CaCl₂ and 50 wt % ofpotassium bicarbonate was mixed with TiO₂ and electrolysed for 20 hours,it also led to full metallisation. The reduction in the two molten saltsproves that the formation of K₄TiO₄ liquid phase is important forincreasing reaction kinetics and is independent of the molten salt used.During electrolysis, all the experiments showed an increase in thecurrent with the inert metallic anode which is in sharp contrast withprevious observations (C. Schwandt [supra]; M. Ma et al., Journal ofAlloys and Compounds 420 (1-2), 37 (2006); and R. O, Suzuki et al,Metallurgical and Materials Transactions B-Process Metallurgy andMaterials Processing Science 34 (3), 287 (2003)).

FIG. 3 displays the current-time plot for the reaction at 3V. Although asmooth curve is observed, there was oscillation in the current with avariation of ±0.1 amps during electrolysis. It can be seen from FIG. 3that there is a decrease in the current for the first half hour of theprocess after which the current increases. Beyond two hours, there is aslow increase in current which plateaus at around 4.0 amps. The largeinitial current is due to the use of the inert anode which has highconductivity compared to a carbon anode and therefore decreases the cellresistance. The initial decrease in current in FIG. 3 is due to theformation of a perovskite phase (verified from the X-ray diffractionanalysis). The data for half hour electrolysis showed the presence ofCaTiO₃, CaTi₂O₄, Ti₃O₅, TiO and Ti metal phases. After 4 hours, almost95% of TiO₂ is reduced to Ti. It is important to note that in previousexperiments (Alexander [supra] and Schwandt [supra]), no titanium metalhas been observed in the first 30 minutes of the process.

From the Ti—O phase diagram, it is known that Ti₃O₅ can never be inequilibrium with Ti from which it is concluded that (at an early stage)two simultaneous reactions occur. The first reaction is the formation ofCaTiO₃, CaTi₂O₄ and Ti₃O₅ which dominates the phase constitution. Thesecond reaction is the decomposition of K₄TiO₄ to form TiO and Ti metal.Since the Magneli phases are more electrochemically conducting and theTi metal is formed in the first hour of electrolysis, an increase incurrent is eminent which is what is found in FIG. 3. The diffractionpattern after one hour of electrolysis showed small peaks of CaTiO₃ andpredominant peaks of Ti, CaTi₂O₄ and Ti₃O₅. It must be noted that theXRD does not show the presence of the potassium titanate phase becauseit is a transitional liquid phase during electrolysis.

The amount of Ti metal produced is not only shown by microstructuralanalysis but also by measuring the weight loss after electrolysis (aspreviously demonstrated by G. Z. Chen et al, Metallurgical and MaterialsTransactions B-Process Metallurgy and Materials Processing Science 35(2), 223 (2004) in the case of electro-reduction of Cr₂O₃ in moltenCaCl₂). After electrolysis of 1 g of TiO₂ pellet for 20 hours, thepellet was washed in water for 24 hours and the weight of the pellet wasmeasured again and was found to be 0.605 g. The theoretical amount of Tiproduced from 1 g of TiO₂ is 0.6 g which is within the error ofexperimental observation thus verifying complete metallisation. FIG. 3shows a low magnification image of the cross section of a fullymetallised TiO₂ pellet which was reduced in a CaCl₂—LiCl moltenelectrolyte. As evident from FIG. 4 a, the layered structure as seen inFIG. 2 is absent throughout the cross section. The inset in FIG. 4 areveals that it has a metallic grey colour with a large number of crackson the surface. There was 20% shrinkage in the pellet from its startingthickness of 5 mm to a final thickness of 4 mm. The corresponding highmagnification image of the inner region (within the hole) is shown inFIG. 4 b. The microstructure has a distinctive Ti metal morphologyobtained from the electro-reduction process and compares well withliterature data (Schwandt et al [supra]). The EDX from this regionconfirms Ti having K^(α), K^(β) and L^(α) peaks. In the EDX spectrumshown in FIG. 4 b, an oxygen peak at 1350 ppm concentration in thereduced Ti metal is not anticipated. The designated oxygen peak is TiL^(α) and not O K^(α).

EXAMPLE 3

FIG. 7 is a schematic illustration of a first embodiment of theelectrolytic cell according to the invention designated generally byreference numeral 1. The electrolytic cell 1 comprises an inert alloyanode 2 and a cathodic basket 3 in a molten electrolyte 6 of CaCl₂containing K₂O. Inside the cathodic basket 3 is a TiO₂ pellet 4 aroundwhich is formed a perovskite layer 5. The cell 1 operates at an appliedvoltage of about 3.1V

EXAMPLE 4

FIG. 8 is a schematic illustration of a second embodiment of theelectrolytic cell according to the invention designated generally byreference numeral 11. The electrolytic cell 11 is deployed forcontinuous metal production.

The electrolytic cell 11 comprises four inert alloy anodes 12 a-d. Inertalloy anodes 12 a-c are mounted in a cathodic vessel 13 containing amolten electrolyte 16 of CaCl₂. The molten electrolyte 16 is fedcontinuously into the cathodic vessel 13 together with TiO₂ powder andK₂O into the feed end 20 and a controlled flow of molten electrolyte 16from the feed end 20 to a discharge end 21 is achieved by a slope in thecathodic vessel 13.

During the continuous flow of molten electrolyte 16, TiO₂ is reduced totitanium sub-oxide. In accordance with the invention, this is only madefeasible by the presence of K₂O. At the discharge end 21, there is adischarge port 22 through which titanium sub-oxide is discharged into acathodic separation vessel 31 which houses inert alloy anode 12 d andcompletes the reduction of titanium suboxide to titanium metal. Titaniummetal is discharged from the discharge outlet 30 and the moltenelectrolyte is recycled to the cathodic vessel 13. To determine the endpoint of the process, the separation vessel 31 is fitted with areference electrode to facilitate the measurement of a current vs timeplot.

EXAMPLE 5

FIG. 9 is a schematic illustration of a third embodiment of theelectrolytic cell according to the invention designated generally byreference numeral 21. The electrolytic cell 21 is deployed forcontinuous metal production.

The electrolytic cell 21 comprises three inert alloy anodes 22 a-choused in a vessel 23 containing a molten electrolyte 26 of CaCl₂. Incontact with a cathode 29 is a plurality of baskets 30 each composed ofa self-supporting mixture of TiO₂ and K₂O. Each basket 30 is mounted ona conveyor which circulates the baskets 30 in and out of the moltenelectrolyte 26 in the direction X.

EXAMPLE 6

FIG. 10 is a schematic illustration of a fourth embodiment of theelectrolytic cell according to the invention designated generally byreference numeral 221. The electrolytic cell 221 is deployed forcontinuous metal production.

The electrolytic cell 221 comprises three inert alloy anodes 222 a-choused in a vessel 223 containing a molten electrolyte 226 of CaCl₂.TiO₂ and K₂O is added to the molten electrolyte 226 to form asuspension. A plurality of cathodic pellets 230 is mounted on a conveyorwhich circulates the pellets 230 in and out of the molten electrolyte226 in the direction X.

EXAMPLE 7

FIG. 7 is a schematic illustration of a fifth embodiment of theelectrolytic cell according to the invention designated generally byreference numeral 1. The electrolytic cell 1 comprises an inert alloyanode 2 and a cathodic crucible 3 made of titanium or titanium alloy.The cathodic vessel 3 is suspended in a molten electrolyte 6 of CaCl₂.Inside the cathodic crucible 3 is a molten mixture 4 of TiO₂ and K₂O. Atsurface A, TiO₂ is reduced to titanium metal and oxide ions aretransported from surface B through the molten electrolyte 6 to the inertalloy anode 2.

1.-27. (canceled)
 28. A process for electrochemical extraction of ametal (M) from a metal (M) oxide comprising: applying a voltage betweena cathode comprising or in contact with the metal (M) oxide and an anodein an oxygen-dissolving molten electrolyte in the presence of an alkalimetal (M^(a)) oxide whereby to form an alkali metal (M^(a)) metallate(M) phase, wherein the alkali metal (M^(a)) oxide forms the alkali metal(M^(a)) metallate (M) phase from a reaction of the alkali metal (M^(a))oxide with a metal (M″) metallate (M) phase.
 29. A process as claimed inclaim 28, wherein the alkali metal (M^(a)) oxide is potassium oxide. 30.A process as claimed in claim 28, wherein the metal (M″) metallate (M)phase is a perovskite (or perovskite-type) phase.
 31. A process asclaimed in claim 28, wherein the diffusivity of oxygen in the alkalimetal (M^(a)) metallate (M) phase is higher than the diffusivity ofoxygen in the metal (M″) metallate (M) phase.
 32. A process as claimedin claim 28, wherein the alkali metal (M^(a)) metallate (M) phase is aliquid.
 33. A process as claimed in claim 28, wherein the alkali metal(M^(a)) metallate (M) phase is M^(a) ₄MO₄.
 34. A process as claimed inclaim 28, wherein the alkali metal (M^(a)) oxide is in admixture withthe metal (M) oxide in or in contact with the cathode.
 35. A process asclaimed in claim 28, further comprising: mixing the alkali metal (M^(a))oxide and the metal (M) oxide and forming the mixture of alkali metal(M^(a)) oxide and metal (M) oxide into a self-supporting mixture.
 36. Aprocess as claimed in claim 28, wherein the alkali metal (M^(a)) oxideis formed in situ by decomposition of a decomposable alkali metal(M^(a)) salt, wherein the decomposable alkali metal (M^(a)) salt is inadmixture with the metal (M) oxide in or in contact with the cathode.37. A process as claimed in claim 36, further comprising: mixing thedecomposable alkali metal (M^(a)) salt and the metal (M) oxide andforming the mixture of decomposable alkali metal (M^(a)) salt and metal(M) oxide into a self-supporting mixture.
 38. A process as claimed inclaim 36, wherein the decomposable alkali metal (M^(a)) salt is analkali metal (M^(a)) bicarbonate.
 39. A process as claimed in claim 36,wherein the metal (M) is one or more metals selected from the groupconsisting of Ti, Nb, Ta, U, Th, Cr, Fe, steel and Zr.
 40. A process asclaimed in claim 28, wherein the metal (M) is one or more metalsselected from the group consisting of Ti, Nb, Ta and Zr.
 41. A processas claimed in claim 28, comprising: applying a voltage between a cathodecomprising TiO₂ in admixture with an alkali metal (M^(a)) saltdecomposable into the alkali metal (M^(a)) oxide and an anode in anoxygen-dissolving molten CaCl₂-containing electrolyte whereby to form aliquid alkali metal (M^(a)) titanate phase.
 42. A conducting electrodecomprising a metal (M) oxide and either an alkali metal (M^(a)) oxidecapable of forming an alkali metal (M^(a)) metallate (M) phase or analkali metal (M^(a)) salt decomposable into an alkali metal (M^(a))oxide capable of forming an alkali metal (M^(a)) metallate (M) phase.43. An electrolytic cell comprising a cathode which comprises or is incontact with a metal (M) oxide and one or more inert anodes in contactwith a fusible or fused oxygen-dissolving electrolyte in the presence ofan alkali metal (M^(a)) oxide.
 44. An electrolytic cell as claimed inclaim 43, comprising a single inert anode, wherein the cathode is acathodic basket in which is carried the metal (M) oxide.
 45. Anelectrolytic cell as claimed in claim 43, wherein the cathode is acathodic vessel which is adapted to facilitate in use continuous flow ofthe fused oxygen-dissolving electrolyte between a feeder end into whichthe fused oxygen-dissolving electrolyte is feedable and a discharge endfrom which the fused electrolyte is dischargeable, wherein theelectrolytic cell comprises a plurality of inert anodes housed in thecathodic vessel between the feeder end and the discharge end.
 46. Anelectrolytic cell as claimed in claim 43, comprising a plurality ofinert anodes housed in a vessel which contains the fusedoxygen-dissolving electrolyte, wherein a mixture of the alkali metal(M^(a)) oxide and metal (M) oxide in contact with a cathode is presentin the form of a plurality of self-supporting elements conveyable in usethrough the fused oxygen-dissolving electrolyte.
 47. An electrolyticcell as claimed in claim 43, comprising a plurality of inert anodeshoused in a vessel which contains the fused oxygen-dissolvingelectrolyte, wherein the alkali metal (M^(a)) oxide and metal (M) oxideare present in the oxygen-dissolving electrolyte in contact with aplurality of cathodic elements conveyable in use through the fusedoxygen-dissolving electrolyte.
 48. An electrolytic cell as claimed inclaim 43, wherein the cathode is a metal crucible containing the alkalimetal (M^(a)) oxide and metal (M) oxide in molten admixture, wherein themetal crucible is suspended in the fused oxygen-dissolving electrolyte.